Advanced multimedia services continue to drive requirements for increasing data rates and higher performance in wireless systems. Current technologies for high performance communication systems, such as those specified by the European terrestrial digital video broadcasting (DVB-T) standard, the Japanese integrated services digital broadcasting terrestrial standard (ISDB-T) and the digital audio broadcasting (DAB) standard, employ communication methods based on Orthogonal Frequency Division Multiplexing (OFDM)
As known to those of skill in the art, multipath interference presents a significant impediment to effective wireless communication Due to different length transmission routes, multiple versions of a transmitted data signal arrive at a receiver with different delays. These variable transmission times can result in inter-symbol interference (ISI) when the different data signals arrive at the receiver simultaneously.
In OFDM multiple sub-carrier systems, a higher rate data signal is divided among multiple narrowband sub-carriers that are orthogonal to one another in the frequency domain Two signals are orthogonal if their dot product is equal to zero Thus, the higher rate data signal is transmitted as a set of parallel lower rate data signals carried on separate sub-carriers.
A received OFDM symbol in an OFDM system generally consists of both data and pilot synchronization information transmitted on the multiple sub-carriers multiplexed together and spanning multiple sample periods. Modulation and demodulation in an OFDM system uses an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver At the transmitter, a cyclic prefix of a section of the IFFT output for each OFDM symbol is typically appended to the beginning of the OFDM symbol as a guard interval (GI) The length of the OFDM symbol before adding the guard interval is known as the useful symbol period duration At the receiver, the cyclic prefix is removed prior to the FIT demodulation by the appropriate positioning of an FFT window, which has a size equal to the useful symbol period duration, along a received sample sequence Subsequently, FIT demodulation transforms the window of received time domain samples, in the received sample sequence, to a frequency domain (OFDM) symbol
As shown in FIG. 1, an exemplary OFDM symbol sequence 100 includes a series of OFDM symbols, each having a useful symbol period duration Tu, appended by a GI of duration Tg For example, GI 101 for OFDM symbol 102 appends a last portion 103 of OFDM symbol 102 at the beginning In a received sample sequence, the location of the GI and useful symbol periods is typically not known at the receiver An estimate of the location of the guard interval may be determined by correlating a first Tg length segment 104 of the received sample sequence with a second Tg length segment 105 separated by the useful symbol period duration Tu Autocorrelation plot 107 reflects the resulting correlation that peaks when the first Tg length segment 104 is positioned over GI 101. The demodulator's FIT window, having a duration Tu, is then positioned to pass the OFDM symbol 102, including the last portion 103, to demodulation while discarding GI 101
A principle advantage of this type of communication system is that the lower data rate occupies a longer symbol period than in a higher rate single carrier system. The addition of the GI to each lower frequency symbol contains the dispersion caused by multipath within the longer symbol period, reducing or eliminating ISI OFDM systems also offer a number of other advantages relevant to wireless applications, including high spectral efficiency and the ability to compensate for poor channel conditions, including signal fade
Further details regarding OFDM systems can be found in co-pending, commonly-assigned U.S. patent application Ser. Nos. 12/272,629, filed Nov. 17, 2008, Ser. No. 12/277,247, filed Nov. 24, 2008, Ser. No. 12/277,258, filed Nov. 24, 2008, Ser. No. 12/365,726, filed Feb. 4, 2009, Ser. No. 12/398,952, filed Mar. 5, 2009, and Ser. No. 12/512,273, filed Jul. 30, 2009 {“System and Method for reducing phase errors in multiple sub-carrier communication systems” by Inventors Hao-Ren Cheng, Kuang-Chung Ou, William McFarland}, all of which are hereby incorporated by reference in their entirety
As will be appreciated, the performance of an OFDM system depends upon the appropriate positioning of the FFT, so that the portion of the signal corresponding to the OFDM symbol is passed on, while the GI is discarded In a multipath environment or in single frequency networks (SFN), such as DVB-T and ISDB-T, multiple, superimposed copies of a sequence of transmitted OFDM symbols arrive simultaneously Correspondingly, each copy of the sequence of transmitted OFDM symbols typically incurs a different time delay and is scaled in amplitude by a different gain.
The GI discussed above adds flexibility to the positioning of the demodulation FIT window to minimize or eliminate ISI and thereby improve performance. For example, if a maximum delay spread between the beginning of the earliest received copy of an OFDM symbol and the beginning of the latest received copy of the same OFDM symbol is less than the length of the GI, the FFT window can be positioned to eliminate ISI Alternatively, if the maximum delay spread is greater than the length of the GI, ISI is not eliminated, but the FIT window may be positioned to minimize the interference. Accordingly, the FFT window must be placed accurately if ISI is to be eliminated or minimized
Prior art methods for positioning the FIT window include strongest signal correlation, first signal above threshold and center of gravity. Samples spaced by an interval corresponding to the useful signal length are taken and compared to generate a correlation signal. As discussed above, a plurality of signals are available to the receiver in a multipath environment or a SFN system The strongest signal technique simply correlates peaks in the strongest received signal to position the FFT window for all the signals The first signal above threshold technique correlates to the first received signal that has sufficient strength. Finally, the center of gravity technique essentially “averages” the received signals and correlates to the imputed center. Unfortunately, none of these techniques work particularly well for a SFN system
Another prior art technique positions the FFT window to maximize the carrier-to-noise ratio (C/I). However, this solution does not provide a satisfactory method for determining signal arrival time. Other prior art techniques use channel estimation information to position the FFT window. Correspondingly, these cannot be applied in communication systems that do not provide scattered pilot symbols, such as DAB and non-coherent ISDB-T Yet other techniques position the FFT window based on computed bit error rates (BER) Such techniques suffer from a long delay between FFT window selection and the corresponding BER determination, particularly when the communication system employs time interleaving Thus, BER-based methods do not offer optimized performance, particularly under rapidly changing conditions such as mobile applications.
Accordingly, it would be desirable to provide systems and methods that position the FFT window to minimize or eliminate ISI, even in multipath environments or SFN systems. Similarly, it would be desirable to provide such systems and methods that do not require channel estimation using scattered pilots and can rapidly provide feedback to optimize window position and improve performance, even under quickly-changing conditions